Method of making a catalyst coated with samarium oxide

ABSTRACT

THIS INVENTION RELATES TO A PROCESS OF MAKING A DECOMPOSITION CATALYST FOR CONCENTRATED HYDROGEN PEROXIDE COMPRISING ETCHING AN ALLOY OF SILVER AND PALLADIUM WITH NITRIC ACID AND THEN COATING THE POROUS ALLOY WITH SAMARIUM OXIDE, OBTAINED BY DECOMPOSING SAMARIUM NITRATE BY HEAT.

Feb. 2, 1971 g, v cco c 3,560,407

METHOD OF MAKING A CATALYST COATED WITH SAMARIUM OXIDE Original FiledJuly 20, 1965 la -V4 SECTION A-A INVENTOR. JAMES C. McCORMl CK BY MUnited States Patent 3,560,407 METHOD OF MAKING A CATALYST COATED WITHSAMARIUM OXIDE James C. McCormick, West Seneca, N.Y., assignor to FMCCorporation, New York, N.Y., a corporation of Delaware Originalapplication July 20, 1965, Ser. No. 473,328. Divided and thisapplication Nov. 30, 1967, Ser. No. 702,764

Int. Cl. B013 11/06 U.S. Cl. 252-462 4 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to a process of making a decomposition catalystfor concentrated hydrogen peroxide comprising etching an alloy of silverand palladium with nitric acid and then coating the porous alloy withsamarium oxide, obtained by decomposing samarium nitrate by heat.

This is a division of application Ser. No. 473,328, filed July 20, 1965.

This invention relates to an improvement in decomposition catalysts andmore particularly to an improved catalyst useful in the decomposition ofhighly concentrated hydrogen peroxide used as a propellant.

Hydrogen peroxide is a known propellant for use in rocket engines,particularly at concentrations of 90% and above. In operation theconcentrated hydrogen peroxide is passed under pressure into one end ofa rocket chamber in contact with a decomposition catalyst. The catalystaccelerates decomposition of the hydrogen peroxide and the resultantgases which are exhausted from the other end of the rocket chamberprovide the desired thrust. Because of the simplicity and reliability ofusing concentrated hydrogen peroxide as a propellant, it has beenextensively used to power small rockets or thrust chambers employed inspace vehicles for attitude control purposes.

In such hydrogen peroxide powered rockets the decomposition catalystmust meet certain criteria in order to satisfy the needs of attitudecontrol rockets. One criterion is that the decomposition catalyst mustdecompose the hydrogen peroxide at a rate sufficient to obtain thedesired thrust. Moreover, this thrust must be reasonably uniform withsucceeding pulses as the rocket is activated.

Another requirement is that the time interval between activation of therocket, i.e., the entry of the peroxide into the rocket chamber, andproduction of the desired thrust, which is sometimes referred to as thestarting transient, must be kept below certain time periods-- generallybelow about 100 milliseconds. Ideally a system having no time delaybetween the entry of the peroxide into the rocket chamber and deliveryof the maximum thrust is desired. A small starting transient isexceedingly important in attitude control of space vehicles becauserapid response to controls are required.

A further requirement of the decomposition catalyst is that it have arapid decay rate after the hydrogen peroxide fuel has been shut off.That is, the hydrogen peroxide decomposition rate must be sufficientlyrapid that little remaining hydrogen peroxide continues to decompose forsustained periods of time after the hydrogen peroxide flow to the rocketchamber has been shut off. A rocket catalyst having a slow decay wouldcontinue to exert a thrust after being deactivated and would constantlyovershoot the correct attitude control point.

Desirably, the catalyst should also have a melting point sufficientlyhigh so that it does not melt after sustained peroxide decomposition orafter numerous intermittent pulses. This problem is more acute whenusing "ice higher concentrated hydrogen peroxide, on the order of about98% than with 90% hydrogen peroxide, since higher decompositiontemperatures are encountered with 98% propellant. This melting is highlyundesirable because it reduces the amount of catalyst surface area whichcan contact the hydrogen peroxide, and lowers the overall performance ofthe rocket. Of the numerous metals which have been employed asdecomposition catalysts silver has generally been found acceptable insatisfying most requirements. However, silver suffers the drawback ofbeing a relatively low melting metal on the order of about 1500 -l600 F.(inthe presence of a high 0 partial pressure generated by decomposing H0 and tends to melt and erode on successive firings, especially when 98%hydrogen peroxide is used.

In certain cases silver has been alloyed with other metals such as goldto form an alloy of 99% Ag and 1% Au in order to improve itsperformance. Such an alloy is described in US. Pat. 3,135,703 issued onJune 2, 1964. However, when silver is alloyed with other metals itscatalytic properties are often adversely affected and any advantagegained with respect to the physical property of the silver alloy isoffset by the loss of catalytic activity.

As a result there is a need for a decomposition catalyst for hydrogenperoxide which can be used with hydrogen peroxide having concentrationsof 90% and above and which can withstand the decomposition conditionsunder which the peroxide is decomposed without adversely affecting itscatalytic activity.

It is an object of the invention to satisfy the above need.

It is a further object of the present invention to provide a catalystuseful in space vehicle attitude rockets which is not adversely affectedby the conditions obtained during rocket firing.

These and other objects will be apparent from the following disclosure.

I have found that a decomposition catalyst containing from about 20 toabout 35% palladium and from about to about silver and coated with adiscontinuous film of samarium oxide is an excellent decompositioncatalyst for hydrogen peroxide and has a long catalytic life even whenused to decompose hydrogen peroxide having concentrations on the orderof about 98% and above. I have further found that the above catalyst haseven improved physical properties if manganese is added to the abovepalladium-silver alloy in amounts up to about 5%, without adverselyaffecting the catalytic decomposition properties of the above definedpalladium-silver catalyst.

The combination of these two metals produces an alloy having all thedesirable catalytic activity of silver but which contains none of thedeficiencies of silver alone. Even greater improvements in the physicalproperties of the catalyst is obtained by adding a small amount on theorder of about 3 to 5% of manganese to the alloy.

In carrying out the present invention an alloy containing the requiredamounts of palladium, silver and preferably manganese is formed intofine wires and woven into screens. The screens are then treated as setforth hereinafter to render them active and are placed in thedecomposition chamber of a rocket. Hydrogen peroxide is fed into thedecomposition chamber through one end of the rocket. The hydrogenperoxide decomposes violently upon contact with the decomposition screenand the decomposition gases are then ejected from the 0pposite end ofthe rocket to supply the desired thrust. The melting temperature of thecatalyst is at least about 2l00 F. and is well above the maximumtemperature obtained in the decomposition of highly concentratedhydrogen peroxide; including 98% hydrogen peroxide.

In general, the starting transient of the decomposition catalyst, thatis the time interval between entry of the peroxide into the rocketchamber and production of the desired thrust, is below about 100milliseconds. Moreover, the decomposition catalyst also has an extremelyand desirably fast decay rate. That is, the time interval between thecut off of peroxide into the rocket and the elimination of substantialthrust can be kept small, generally below 100 milliseconds.

While the decomposition catalyst described above is in the form of solidalloy, it should be understood that only the outer surface of the alloycontacts the hydrogen peroxide catalyst. Thus it is perfectly feasibleand, in fact, economically desirable to make up the catalyst by placinga coating of the catalyst alloy on a high temperature base materialwhich can resist the conditions fo the decomposition catalyst. Anexample of a suitable base is nickel which has a melting point above2400 F.

The coated catalyst can be made up by flame spraying the alloy onto anickel wire. Flame spraying is accomplished by passing the alloy througha high temperature flame and depositing the melted alloy as a firmlyadhered coating on the nickel wire. Thereafter, the nickel wire, coatedwith the catalyst alloy, can be woven into a screen, activated as setforth hereinafter, and placed in the decomposition chamber of a rocket.The flame spraying technique is desirable because it yields a firmlyadhered layer of catalytic alloy to a nickel base. However, other knowntechniques may be used to coat the catalyst alloy on nickel basescreens. For example, the alloy can be deposited by vapor deposition orby dipping the nickel in a molten bath of the metal alloy.

The catalyst screen, whether made from solid wires of the catalyst alloyor from a nickel base coated with the alloy, must be activated beforebeing used in the rocket. The activation comprises, first treating thescreen in a manner to roughen the surface of the individual wires. Thisis done, preferably, by sand blasting the screen although other meanscan be used. Thereafter, the screen is dipped in nitric acid, preferablyhaving a concentration of 20% to 70% by weight. The nitric acidtreatment dissolves a trace of the metal alloy. During this dip, it ismost important that the nitric acid contact all of the abraded surfacesof the decomposition alloy. Thereafter the screen is removed from thenitric acid and the wetted screen is heated at temperatures of at least500 F., but below the melting point of the screen until it is perfectlydry. The screen is then dipped in a 2 to 20% by weight aqueous samariumnitrate solution and then heated to at least about 900 F. for at leastabout 5 minutes. This procedure is repeated until a discontinuous filmof samarium oxide is formed on the surface of the screen. Thereafter thescreen is cooled and placed in the catalytic chamber for use.

The samarium oxide discontinuous film is used on the catalyst to preventlarge gas bubbles generated during decomposition of the peroxide fromadhering to the surface of the catalyst. The samarium oxide film appearsto prevent large gas bubbles from being evolved and building up intopockets surrounding the catalyst. These gas pockets are most undesiredsince they prevent free access of the peroxide to the catalyst metal andthus decrease the effectiveness of the catalyst.

The invention will now be illustrated by reference to the drawings. Inthe drawings:

FIG. 1 is a longitudinal sectional view of a rocket equipped with acatalyst bed constructed in accordance with the present invention.

FIG. 2 is a sectional view throuhg the rocket.

In FIG. 1 the rocket chamber 2 includes a mounting flange 4, a catalystpack 6 and exhaust port 8. The hydrogen peroxide enters inlet port 10and strikes a dispersion plate 12 where it is distributed uniformlythroughout the cross section of the catalytic pack 6. The hydrogenperoxide then enters the catalytic pack 6 where it is decomposed bycatalyst 20 and the evolved gases escape through a perforated plate 14which performs the dual function of retaining the catalytic bed in placewhile permitting the decomposition gases to escape through exhaust port8. The gases leaving the exhaust port provide the thrust to the entirerocket. In general, the hydrogen peroxide is fed into inlet port 10under high pressure suflicient to prevent any decomposition gases frombacking up into inlet port 10. In rocket chamber 2 the catalytic alloy,in the form of wires woven into screens, is placed in stacks. Thesestacks of wire mesh disks constitute the catalytic pack 6 and can beremoved from the chamber after its life has been exhausted by removingretaining screws 16 which hold both mounting flange 4 and retaining ring18 in a locked position in the rocket chamber. Retaining ring 18 mustfirst be removed before mounting flange 4 can be Withdrawn from the headof the rocket. An exhaust orifice 22 is shown connected to exhaust port8 for measuring the pressure that builds up in exhaust port 8. Thedifference in pressure between inlet port 10 and exhaust port 8indicates the reaction efficiencies of the catalyst under identicaltesting conditions.

The following examples are given to illustrate the present invention andare not deemed to be limiting thereof.

EXAMPLE 1 A series of one-inch diameter screens, 20 x 20 inches made upof .014 inch diameter pure nickel wire were annealed by heating them ina furnace to a temperature of 1400 F. and then allowing them to cool inthe deactivated furnace overnight. The wire screens were then flamesprayed with an alloy of 70% by weight silver and 30% by weightpalladium. The flame spraying was accomplished by feeding the alloy inwire form (.057 inch in diameter) into an oxygen-acetylene flame anddepositing the melted alloy onto the wire screens until a uniformcoating was obtained. The alloy coatings were found on an average to beabout .003 inch thick. The screens were activated by sand blasting themto roughen the surface of the wires, dipping them in a 70% HNO solutionand heating the screens to about 500 F. The screens, after cooling, weredipped into a 10% by weight samarium nitrate-water solution and thenheated at 1000 F. for 5 minutes. This was repeated five times until adiscontinuous film of samarium oxide was formed on the wires. Thescreens were assembled into a catalytic pack and placed in the catalyticchamber of a rocket constructed as set forth in FIG. 1. The rocket wasthen mounted on a stand, and a source of 98% hydrogen peroxide wasconnected through a flexible hose to the inlet port of the rocket. Asolenoid valve in the hose controlled the flow of peroxide into therocket. The hydrogen peroxide was fed into the rocket at 350 p.s.i. andthe pressuresdeveloped in the exhaust port 8 during the pulse firingswere recorded over the period of the pulse. The thrust of the rocket wasalso recorded. The test procedure was as follows:

(a) three, 200 millisecond pulses, with a 3 second interval betweenpulses;

(b) 10 minute delay, followed by two 150 millisecond pulses per secondfor 300 seconds;

(c) allow motor to cool, and repeat step (a);

(d) two 150 millisecond pulses per second for 6 seconds;

(e) repeat steps (c) and (d).

During step (a) the second pulse gave a total pressure in the exhaustport of 184 p.s.i.g. and 90% of this pres sure was developed in 90milliseconds. The total thrust was 29.9 lbs. After the peroxide flow wasstopped of the exhaust port pressure developed was lost after 60milliseconds, indicating an 80% decay in 60 milli seconds.

During step (a) the third pulse gave a total exhaust port pressure of214 p.s.i.g., and of the pressure was developed in 68 milliseconds. The80% decay time was 42 milliseconds.

Thereafter, the succeeding pulses gave a total exhaust port pressure of225 p.s.i.g., and 90% of the pressure was developed in 40 milliseconds.The 80% decay time was 30 milliseconds. The thrust of the rocketcontinued at about 30 lbs. The specific impulse (pound seconds of thrustper pound of propellant) was 149 seconds.

EXAMPLE 2 The test procedure of Example 1 was repeated with a secondcatalyst alloy, 75% silver, 20% palladium and manganese (all percentsare by weight). In this test the 98% H 0 was fed into the rocket at apressure of 350 p.s.i.

The'pulses in steps (b) to (e) gave a total pressure in the exhaust portof 262 p.s.i. and 90% of this pressure was developed in 64 milliseconds.The 80% decay time was 42 seconds and the total thrust of the rocket was31.6 lbs. The specific impulse (pound seconds of thrust per pound ofpropellant) was 144 seconds.

EXAMPLE 3 Example 1 was repeated with catalyst screens, 20 x 20 meshmade up of 0.14 inch wires fabricated from an alloy containing 70% byweight silver and 30% by weight palladium, and activated as set forth inExample 1. The results were substantially the same as those obtained inExample 1.

While the catalyst bed of the present invention is intended principallyfor use in rockets or thrust chambers it should be appreciated that thepresent invention is equally applicable to any unit in which hydrogenperoxide must be decomposed quickly, for example, a gas generator,turbo-pump drive or as an oxidant in bipropellent rockets or engines.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a manner so that itcan be readily practiced by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be clearly understoodthat, within the scope of the appended claims, the invention may bepracticed by those skilled in the art, and having the benefit of thisdisclosure otherwise than as specifically described and exemplifiedherein.

What is claimed is:

1. Process of producing a decomposition catalyst for decomposingconcentrated hydrogen peroxide in a thrust chamber which comprisesmaking up a metal alloy containing 65% to 80% by weight of silver and20% to 35% by weight of palladium, roughening the surface of said metalalloy, contacting said metal alloy with nitric acid having aconcentration of from about 20% to about 70% by weight until a portionof said metal alloy dissolves, thereafter heating the thus treated metalalloy to a temperature of at least about 500 F. but below the meltingpoint of the alloy, contacting the metal alloy with an aqueous samariumnitrate solution, heating the metal alloy covered with samarium nitrateto a temperature of at least about 900 F. to convert said samariumnitrate to samarium oxide, and recovering the metal alloy having adiscontinuous coating of samarium oxide thereon.

2. Process of producing a decomposition catalyst for decomposingconcentrated hydrogen peroxide in a thrust chamber which comprisesmaking up a metal alloy containing to 80% by weight of silver and 20% to35% by weight of palladium, depositing a coating of said metal alloyonto a nickel surface, roughening the surface of said metal alloycoating. contacting said metal alloy coating with nitric acid having aconcentration of from 20% to about by weight until a portion of themetal alloy is dissolved, heating the thus treated metal alloy to atemperature of at least 500 F. but below the melting point of the metalalloy coating, contacting the metal alloy coating with an aqueoussamarium nitrate solution, heating the metal alloy coating covered withsamarium nitrate to a temperature of at least about 900 F. to convertsaid samarium nitrate to samarium oxide, and recovering a nickel-basedcatalyst having said metal alloy coating and a discontinuous coating ofsamarium oxide on said metal alloy coating.

3. Process of claim 2 wherein said metal alloy contains up to about 5%by weight of manganese.

4-. Process of claim 2 wherein said metal alloy is deposited on saidnickel by passing said alley through a flame at a temperature sufiicientto melt said alloy and depositing said melted alloy as a coating on saidnickel.

References Cited UNITED STATES PATENTS 2,004,141 6/1935 Tilley et al.252474X 2,802,889 8/1957 Frevel et a1. 252474X 2,838,462 6/1958 Pease252474X 2,927,141 3/1960 Cohn et al. 252474X 3,135,703 6/1964 Sill252474 3,212,255 10/1965 Putt et al. 252474X LELAND A. SEBASTIAN,Primary Examiner U.S. Cl. X.R.

